"I have written about the true grace of God." Here the word "grace" refers to the gospel message, which tells of the kind things that God has done for believers. (See: [[rc://en/ta/man/translate/figs-metonymy]])
The word "it" refers to "the true grace of God." Being strongly committed to this grace is spoken of as standing firmly in one place, refusing to move. Alternate translation: "Remain strongly committed to it" (See: [[rc://en/ta/man/translate/figs-metaphor]])